1. Field of the Invention
This invention relates to a method of producing a corrugated tube made of synthetic resin, and more particularly to a method of manufacturing, on a continuous basis, a corrugated tube the outer wall of which is corrugated in longitudinal cross-section and the inner wall of which has a smooth surface.
2. Prior Art
As is well known, tubes made of synthetic resin are widely used as various structural members, piping, drainage pipes and the like because they have higher corrosion-resistance, chemical resistance and non-magnetic properties in addition to being of light weight and having higher flexibility, as compared to metal tubes. On the other hand, such synthetic resin tubes have the disadvantage that they provide less crushing strength against exterior pressure due to their flexibility. If they are buried as drainage pipes in the earth, they are susceptible to deformation and failure under an external pressure.
As one of approaches to overcoming the foregoing drawbacks, synthetic resin tubes having tube walls corrugated so as to enhance their crushing strength against exterior pressure have been developed and heretofore put to practice in many applications.
These flexible convoluted or corrugated synthetic resin tubes have either convolutions helically extending or convolutions symmetric with respect to the axis of the tube. In either case, such convolutions are formed by shaping concave-convex or undulating formations in the tube wall, and corrugated tubes may be provided with some degree of crushing strength while maintaining necessary flexibility or pliability by selecting the depth and pitch of the convolution, the thickness of the tube wall, etc., apart from the selection of the material of which the tube is formed. Nevertheless, the corrugated tube may not offer strong resistance when crushed by an exterior pressure as it tends to expand or contract between adjacent convolutions for the lack of any solid connection between the adjacent convolutions. This is true particularly when the tube has an enlarged diameter or when adequate flexibility is required of the tube.
In addition, when the corrugated tubes having undulating tube walls necessarily formed by corrugations are used as drainage pipes, for example, the crests of the corrugations provide obstructions to the flow of liquids while deposits tend to collect in the valleys of the corrugations, thus resulting in greatly deteriorating the function of the drainage pipe.
A method of producing a corrugated tube having a convoluted outer wall and a smooth inner wall is known as per U.S. Pat. No. 3,671,348. According to this known method, a pair of strip members of synthetic resin are extruded at the same speed with a profiling insert such as a rigid hollow member or a rope for forming convolutions sandwiched between and heat sealed to the strip members to form a single ply strip having a profiling insert embedded therein and extending longitudinally therealong. This strip is then helically wrapped onto a rotating mandrel while moving the mandrel in the axial direction such that the edges of the adjacent turns of the strip are overlapped and welded together. When the strip is wrapped around the mandrel along its full length to form a length of corrugated tube, the tube is removed from the mandrel. However, this method requires the use of two strip members and complicated operation and control for extruding the two strip members at the same speed and embedding an insert therebetween. Furthermore, in this method the strip is wrapped around the mandrel while displacing the latter, so that only a length of corrugated tube corresponding to the length of the mandrel can be produced in one operation. Moreover, great difficulties are involved in withdrawing the finished corrugated tube from the mandrel. The longer the mandrel, the more difficult is it to remove the tube from the mandrel. For these reasons, the method described above is not a practical method.
In addition, when it is desired to change the wall thickness of the finished tube, the dies for extruding strip members must be replaced with different dies for extruding thicker or thinner plastics strip members, which is a time-consuming procedure.
Still another disadvantage is that since two strip members are welded together by a profiling pressure roller with a profiling insert embedded between the strip members, the convolutions may be collapsed or otherwise improperly formed unless the insert is sufficiently rigid.